host fst - return a better error for ChangePathGS when destination file exists and DestroyGS when file is non-empty directory

src/host_common.c

@@ -256,6 +256,9 @@ word32 host_map_errno(int xerrno) {
       return outOfMem;
     case EEXIST:
       return dupPathname;
+    case ENOTEMPTY:
+      return invalidAccess;
+
     default:
       return drvrIOError;
   }

src/host_fst.c

@@ -1527,7 +1527,9 @@ static word32 fst_change_path(int class, const char *path1, const char *path2) {
   if (host_is_root(&st))
     return invalidAccess;
 
-  // rename will delete any previous file.
+  // rename will delete any previous file. ChangePath should return an error.
+  if (stat(path2, &st) == 0) return dupPathname;
+
   if (rename(path1, path2) < 0) return host_map_errno_path(errno, path2);
   return 0;
 }